The transcriptional organization of simian virus 40 (SV40) will be determined during lytic infection and in transformed cells using an ultraviolet light (UV) mapping technique. Since UV induced photoproducts in DNA cause premature termination of transcription, the UV sensitivty of the expression of a particular gene is a measure of the distance of that gene from the transcriptional start point (promotor). The UV sensitivities of the expression of SV40 early antigens (T and U) will be determined, thereby defining the locations of the promotor(s) utilized in their transcription. By irradiating either the virion or the infected host cells and then measuring the effect of the UV irradiation on SV40 gene expression, it will be possible to determine whether transcription of SV40 genes is initiated within the viral genome or whether it is initiated within the host genome (for transcription of integrated viral genomes). Furthermore, by comparing the UV sensitivities of the expression of SV40 genes measured during lytic infection with those measured in transformed cells, it will be possible to determine whether the transcriptional patterns of the early genes are the same or different for the two viral states. The transcriptional organization of the SV40 late genes will also be determined using the UV mapping technique. Synthesis of late proteins will be assayed by electrophoresis of radioactively labeled proteins on polyacrylamide gels. The UV mapping technique will be critically tested in the SV40-eukaryotic cell system with the expectation that it will be applicable also to the study of transcription in other DNA tumor viruses and eventually in eukaryotic cells themselves.